The limited dynamic range of traditional interference cancellation systems has prevented them from affording full receive system sensitivity when high power cosite signals are present. High power interfering signals are almost always accompanied by noise and distortion effects which traditional cancellers are unable to eliminate. Fielded systems have typically ignored these secondary destructive components caused by distortion of the interfering signal which may present at the desired receive frequency and may desensitize the receiver. Such on-channel interference components, the noise and intermodulation products generated within either the transmitter or receiver, can easily remain well above the receive system sensitivity threshold thereby significantly desensitizing the receiver.
Three distinct interfering signal issues should be successfully addressed in order for an interference cancellation system to achieve essentially full cancellation and enable the receiving equipment to provide “full” sensitivity performance.
First, receiver overload must be prevented. Legacy interference cancellation system designs have traditionally accomplished this for signal levels on the order of 0 dBm but remain incapable for significantly higher levels.
Second, secondary noise and distortion components generated within the transmitter must be eliminated. A strong signal is likely to be accompanied by significant noise and distortion components, such as “phase noise” and intermodulation distortion, which can prevent full receive sensitivity performance. Neither analog nor digital interference cancellation systems have traditionally accomplished the task of eliminating such residual noise and distortion. Analog interference cancellation systems do not have the dynamic range needed due to internal generation of both noise and intermodulation distortion when large signals are present. Digital interference cancellation systems are even more restricted in dynamic range due to inherent limitations at both the analog to digital and digital to analog interfaces.
Third, internally generated noise and distortion within signal-cancelling circuits must be eliminated. The legacy approach to minimizing internally generated noise has been to provide the cancellation system an undistorted digital image of the interfering signal which is being transmitted. Besides the obvious interfacing complexity, this technique has the disadvantage that on-channel distortions within both the analog transmission chain and in the receiving cancellation circuits will still remain to degrade sensitivity even after the undistorted interfering signal components have been removed.
Therefore, a need remains for a system and method enabling large signal cancellation contemporaneous with full system sensitivity for small signal throughput.